On new construction, I have been having the top of my showers tiled for looks. I am buying a standard 4 ft shower(fiberglass). I have been having tile installed from the top of the shower up towards the ceiling. Gives a dressy look. The problem is where the drywall goes over the lip around the top of the shower. This make drywall have a slight arc in it. When you tile it is very noticeable that the tile is not running straight up or square...any suggestions on this...Some have suggested putting lath strips on wall to bring wall out over flange..this is ok but not if the wall is large. Can you just build up the mastic on the top and have it thin where the drywall goes over the flange trying to make the tile flat???